High-frequency electrical transmission system



. Jan. 11, 1938. c, WHITE 2,105,305

HIGH FREQUENCY ELECTRICAL TRANSMISSIQN SYSTEM Filed Aug. 50. 1935Patented Jan. ll, 1938 HIGH-FREQUENCY ELECTRICAL TRANS- MISSION SYSTEMEric Lawrence CaslingWhite, Hillingdon, England, assignor to Electric &Musical Industries Limited, Middlesex, England, a company of GreatBritain Application August 30, 1935, Serial No. 38,529 In Great BritainAugust 31. 1934 7 Claims.

The present invention relates to the transmission of high-frequencyelectric currents, such as 10 two wires of an open wire line. Disturbingpotentials may occur between the two conductors in parallel and earth,owingto conduction currents in the earth or by induction fromneighbouring conductors carrying other currents, e. g. power 15 lines.

In order to obtain only the potential difference which is required, thereceiving apparatus may be connected only to the two conductors, andinsulated from earth, but it is often essential to Q earth some part ofthe apparatus, e. g. when the receiving apparatus is a wirelesstransmitter. It is not possible to earth one of the conductors, even thesheath of a concentric cable, since the interfering voltage would thenbe partly transg ferred to give a. potential difference between the twoconductors.

In telephone practice this difliculty has been overcome by connectingthe primary of a transformer to the line, insulated from earth, and con-9 necting the secondary to the receiving apparatus,

any part of which can then be earthed. However, it is not feasible touse transformers in television work, owing to the very wide frequencyband to be covered with no appreciable phase dis- 35 placement.

An object of the present invention is to provide means for reducing oreliminating the undesired potentials in a terminal station of thetransmission line, while permitting a part of the apparatus at thisstation to be connected to earth.

According to the present invention there is provided a high frequencyelectrical transmission system comprising a transmission line having twoconductors coupled to a terminal station without 1 the use of atransformer wherein the terminal station includes a thermionic valve orvalves having a part of the electrical circuit associated therewithconnected to earth, and is of such a nature that whilst the signalpotential differences 60 existing between the line conductors areapplied to and transmitted by the valves without appreciable phasedisplacement or with almost exact phase reversal, neverthelessinterfering potential diiferenoes existing between the line conductors,in parallel, and earth are prevented or largely prevented frominfluencing the output of the valve or valves. The terminal station maybe so arranged as to prevent or largely prevent the interfering signalsfrom being applied to the input valve of the terminal station.Alternatively 5 valves of the terminal station may be so connected tothe two line conductors and to earth that the interfering signals areapplied to the valve system in such a manner as to be substantiallyeliminated thereby. 10

Thus the line conductors may be connected together by a terminatingresistance across which are developed the signal potentials to beapplied to a valve the cathode of which is connected to earth through animpedance which forms part of the output circuit of the valve, and whichis of such a high value relative to the terminating and line resistancesthat only a low potential is developed across the terminating resistanceby the interfering potential differences, while the value of theimpedance is low enough relative to the anode slope resistance of thevalve to prevent appreciable variations in the anode current, due to theinterfering potentials developed across the impedance. 5

Alternatively the line conductors may be connected to the control gridsof two valves whose cathodes are connected to earth, one of these valvesbeing associated with a phase-reversing device the output circuit ofwhich is in parallel with the output circuit of the other valve, and.the arrangement being such that interfering signals appear in oppositephase in the output circuits and cancel out, while the desired signalsappear in like phase.

It is to be understood that, throughout this specification, a'connectionto earth does not imply a direct connection to earth but may imply aconnection to a point the potential with respect to earth of whichissubstantially incapable of 40 variation at any frequency within therange of frequencies which the system is adapted to handle or is onlycapable of variations which have no adverse effect on the operation ofthe system.

The invention will be described by way of example with reference to theaccompanying diagrammatic drawing in which Figs. 1 and 2 show twoarrangements according to the present invention. 5

Referring to Fig. 1 a two wire line I comprising two conductors 2, 3 isconnected at one end to a transmitter denoted ml and at the other end toa terminal station. Conductor 3 is earthed at a point adjacent thetransmitter 4. In the terminal station the conductors 2, 3 areterminated by a terminating resistance 5 which may have a value equal tothe characteristic impedance of the line I. Conductor 3 of the line I isconnected to the cathode 6 of a thermionic valve 1 through a largecondenser 8, for isolating steady potentials, and a grid bias battery 9.The other conductor 2 is connected to the control grid III of the valve1 through another isolating condenser II. A high resistance grid leak I2is provided to maintain the required steady potential on the grid illfrom the battery 9. The cathode 6 of valve I is to a large extentisolated from earth by a large impedance I3, which may be a resistanceof the order of 1000 times the terminating resistance 5, but whichshould be small compared with the anode slope resistance of the valve.For example, if the terminating resistance 5 is 100 ohms and the valveis a tetrode or pentode having an anode impedance of 10 ohms, theearthing resistance 13 may be 50,000 ohms.

The anode circuit may include an anode resistance l4 and a battery 15with one end earthed, which supplies the anode voltage. If the valve 1is a tetrode (as shown) or a pentode, a separate battery l6, insulatedfrom earth, supplies the screen potential.

Differences of potential between the two conductors 2, 3 are amplifiedby the valve 1 and appear across the anode resistance l4, and may betaken off via a condenser I! to an output circuit denoted by IT.

Since conductor 3 is earthed at the transmitting end of the line,diflerences of earth potential between these two stations will causecurrent to flow through line conductor 3, which is connected to thecathode 6. Since the earthing resistance I3 is large, very littlepotential difference will be produced between the ends of line conductor3, and still less potential diflerence will occur across the terminatingresistance 5 due to current flowing through line conductor 2 andresistance 5. The potential developed across the earthing resistance l3will cause very little variation of anode current, owing to therelatively high value of the anode slope resistance of the valve 1.

' Thus the interference is considerably less than that which would occurif the receiving end. of line conductor 3 where connected directly toearth, that is if the earthing resistance l3 were made zero.

In the arrangement of Fig. 2 the receiving ends of the line conductor 2,3 are bridged by a terminating resistance 5, and directly connectedrespectively to the control grids l8, I3 of two input valves 20, 2| thecathodes 22, 23 of which are connected to earth through a biasingresistance 3|. Valve 2! is coupled by a resistance-capacity couplingcomprising an anode resistance 241, a coupling condenser 25 and a gridleak 26 to a third valve 21 whose cathode 28 is connected to earththrough a bias resistance 32 shunted by a decoupling condenser 33, andthe output circuits of input valve 20 and of the third valve 21 areconnected in parallel by means of a common anode resistance 29, which ispreferably very small relative to the anode impedances of the valves 20,21. The common output voltage of valves 20, 21 may be transferred by acoupling condenser 30 to an output circuit denoted by 30'.

The anode potentials for the three valves are furnished by a. battery l5having its negative pole connected to earth.

The product or the voltage amplification of the input valve 2| and ofthe third valve 21 coupled thereto is arranged to be numerically equalto the amplification of the other input valve 20 alone, but is reversedin phase owing to the use of two valves as against one. Care must betaken to avoid introducing any phase differences other than theintentional 180.

Thus equal potential differences between earth and. the two ends ofconductors 2, 3 of the transmission line I, due to interference, willproduce at the anodes of input valve 20 and of the third valve 21impulses which are in opposite phase and which therefore cancel outwhile differences of potential between the two line conductors 2, 3 willproduce at these anodes signals which add in the same phase.

The transmission line I may be a concentric tube cable, or an open wireline,.as already suggested. In either case it is important that bothconductors be insulated from earth. Alternatively, the cable may consistof two concentric tube cables lying side by side, with the sheathsconnected together and earthed at both ends. The two inner conductorsform the line conductors, and at the transmitter end either they may befed in push-pull, or one only may be fed, and the other left as a dummysimply for providing equal interference to that picked up on the workingcable, to be balanced against the latter at the receiving end.

To ensure sufliciently low losses at the modulation frequencies commonin television, e. g., up to 3 x 10 cycles per sec., the dielectric ofthe concentric tube cable may consist of beads of glass or a suitableceramic material, or may consist of hollow insulating shells ofsynthetic resin or similar material, providing supports for 'the centralconductor at intervals of an inch or so.

A further type of cable which can be used in conjunction with thepresent invention comprises a conductor bent to a zig-zag shape andlocated within an insulating tube provided with conducting sheath.

I claim: I

1. A high frequency electrical transmission system comprising atransmission line having two conductors, a source of electrical signalvariations connected between said conductors, said system being of thekind that spurious potential diflerences can be imposed between saidconductors and ground by uncontrolled outside agencies, a

thermionic valve terminal circuit having two input terminals arranged ata point remote from said source, non-inductive coupling means arrangedbetween said conductors and said input terminals, an output circuithaving a point therein connected to ground, and means forming part ofsaid terminal circuit for feeding to said output circuit signalvariations derived from said source substantially free from saidspurious potential differences.

2. A high frequency electrical transmission system comprising atransmission line having two conductors, a source of electrical signalvariations connected between said conductors, said system being of thekind that spurious potential differences can be imposed between saidconductors and ground. by. uncontrolled outside agencies, a thermionicvalve terminal circuit arranged at a point remote from said source, andan output circuit having a point therein connected to ground, saidterminal circuit comprising a terminating impedance element connected inshunt across said line, a thermionic valve having a cathode, a controlgrid and an anode, non-inductive coupling means between said conductorsand said cathode and control grid respectively, a second impedanceelement connected between said cathode and ground, and coupling meansarranged between said anode and said output circuit, the impedance ofsaid second impedance element over the rangeof frequencies covered bysaid electrical variations being large compared with the resultantimpedance due to the shunt impedance of said line and said terminatingimpedance element, so that the potential difference developed acrosssaid terminating impedance element due to said spurious potentialdifferences is small compared put circuit.

3. A high frequency electrical transmission system according to claim 2,wherein over said range of frequencies theimpedance of said secondimpedance element is greater than ten times the impedance of saidterminating impedance element.

4. A high frequency electrical transmission system according to claim 2,wherein over said range of frequencies theanode slope resistance of saidvalve is greater than ten times the impedance of said second impedanceelement.

5. A high frequency electrical transmission system comprising atransmission line having two conductors, a source of electrical signalvariations connected 'between said conductors,

said system being of the kind that spurious po tential differences canbe imposed between said conductors and ground by uncontrolled outsideagencies, a thermionic valve terminal circuit arranged at a point remotefrom said source, and an output circuit having a point therein connectedto ground, said terminal circuit comprising first and second thermionicvalves, means comprising a non-inductive coupling for feeding variationsfrom said line to said first and second valves in push-pull,phase-reversing means for reversing the phase of the output of saidfirst valve, and means for feeding to said output circuit in parallelthe output of said second valve and the output of said phase-reversingmeans. 7

6. A high frequency electrical transmission system according to claim 5,wherein said phasereversing means comprise a further thermionic valvehaving input and output circuits, said input circuit being coupled tothe output circuit of said first valve and the output circuit of saidphase-reversing means being connected in parallel with the outputcircuit of said second valve.

7. A high frequency electrical transmission system according to claim 5,wherein said phasereversing means comprise a further thermionic valvehaving input and output circuits, said input circuit being coupled tothe output circuit of said first valve and the output circuit of saidphase-reversing means being connected in parallel with the outputcircuit of said second valve,

the total amplification of said first valve and said further valve beingsubstantially numerically equal to the amplification of said secondvalve.

ERIC LAWRENCE CASLING WHITE.

